This invention is directed to a gate for employment in the molding of dental prostheses by the lost wax molding process. The gate starts as a wax master having a feeder of square cross section for reduced stress in the investment material with a circular disc on each end thereof. The discs are attached to the prostheses and the reservoir bar. The disc shape aids filleting at these attachments.
In the conventional formation of dental prostheses, a wax master is formed of the dental part to be created. This wax master is mounted by means of a gate upon a reservoir bar. The reservoir bar, in turn, is usually connected through two feeders to a funnel-like sprue. The material from which the gate, reservoir bar and feeders are made is extruded of wax with a circular section. This rod or wire is mounted on a spool. It is provided in several different diameters so that the laboratory technician can select the appropriate cross section. He cuts a reservoir bar from the long length on the spool and cuts usually two feeder bars of the same or slightly smaller size. The feeder bars are connected to the reservoir and are connected to the sprue by means of heating sticky wax and flowing it into the joints.
A plurality of wax masters representing the metal prostheses are attached to the reservoir bar in a similar way. The wax gate extrusion of circular section is cut to length by the technician and secured to both the wax master of the prosthesis and the reservoir bar. When the wax master is complete, investment material is cast around it, the wax is removed, and liquid metal is poured into the sprue. The liquid metal moves through the reservoir bar and gates into the mold recess of the prosthesis and cools therein. As the casting of the prosthesis cools and solidifies, it is supposed to draw liquid metal from the reservoir bar and sprue through its gate into the casting. As the solidifying metal in the casting shrinks, the gate must supply molten alloy to fill in for the shrinkage of the metal. If the metal in the gate solidifies before the center of the prosthesis casting, shrinkage porosity occurs in the center of the prosthesis casting. When the gate is too long, which places the prosthesis casting recess too far from the reservoir bar, the gate will freeze before the casting. Since the present material of circular section is of indefinite length, it is the technician who is required to make the judgment of gate length when he cuts it from the reel. As a result, the gate length is inconsistent, and the importance of a short gate length is not sufficiently recognized. This is particularly true when the prosthesis is a bridge of multiple elements and the bridge is curved. In this case, the technician adjusts the gate length so that the curved bridge is fed from a straight reservoir bar. With a different gate length on each portion of the bridge, poor casting may result.
Another problem which arises from the use of gates and feeders of circular section is that when such lie adjacent each other, the facing curved surfaces produce stress raisers in the investment so that the investment cracks between adjacent gates. In usual dental laboratory practice, this happens quite often. The result is a fin of metal which joins the adjacent castings, gates and the reservoir bar. This fin ruins the castings or requires a great deal of cleanup. The rod material of circular section, supplied in great length for use to make gates, reservoir bars and feeders, is supplied in different diameters for different purposes. As noted above, the gate feeds molten metal to the casting as it cools and, as a result, larger gates are required for larger castings. For a cap for a lower tooth, a 10-gauge gate diameter is sufficient. For a pre-molar cap or an upper anterior tooth cap an 8-gauge gate is proper to feed the mold recess. Furthermore, for a cap for a posterior tooth, which is quite large, a 6-gauge gate would be appropriate to feed the molten metal from the reservoir bar. After the casting is hard and the investment material is removed, the casting must be cut free from the reservoir bar. With different gate diameters, it is difficult to judge the end of the casting, where the cut is to be made to remove the gate and not cut into the casting, and when the cutter disc is used to cut free the casting, occasionally the casting is cut instead of the gate. This requires a new casting which reduces productivity.